Methods are well known in the art for using a tubular reactor to form low density ethylene-based polymers from ethylene, and, optionally, one or more comonomers, such as low density polyethylene (LDPE). The overall process is a free-radical polymerization in a tube reactor containing a process fluid, where the process fluid partially comprised of ethylene and the ethylene is converted to an ethylene-based polymer in a highly exothermic reaction. The reaction occurs under high operating pressure (about 1000 bar to 4000 bar) in turbulent process fluid flow conditions at maximum temperatures of about 160° C. to about 360° C. The reaction initiation temperature, or the temperature in which the monomer (and optional comonomer) to polymer conversion is initiated (or in the case where there are multiple reaction points along the reaction tube, reinitiated), is from about 120° C. to about 240° C. Typical single-pass conversion values for a tubular reactor range from about 20 to about 40 percent.
The reaction is initiated (and reinitiated) by injecting an initiator into at least one reaction zone within the reactor tube. The initiator is mixed with the process fluid and, in the presence of heat (usually latent heat—the process fluid is typically already at an adequate reaction temperature), the initiator forms free-radical decomposition products. The decomposition products start a free-radial polymerization reaction with the ethylene (and optional comonomers) to form the product ethylene-based polymer.
The reaction generates a large quantity of heat in the reaction zones. Without proper cooling, the adiabatic temperature rise in the process fluid (which now contains product ethylene-based polymer that absorbs and retains heat) eventually results in unfavorable and possibly uncontrollable reactions. Such undesirable reactions may include ethylene decomposition (forming products such as carbon, methane, acetylene, and ethane), formation of high molecular weight polymer chains, and termination by combination and crosslinking, which may lead to a broadening of molecular weight distribution. The results of such undesirable reactions range from varying product quality and consistency issues to reaction system shutdown, venting, and cleanup.
Undesirable reactions may also occur when there is inadequate distribution of initiator in the process fluid. Under normal process operating conditions, initiator quickly breaks down into free-radical products after being injected into the process fluid. Dispersion of the initiator into the process fluid often results in a localized zone of high initiator concentration inside the process fluid flow. This localized initiator zone fosters an unbalanced reaction profile in the process fluid: greater amounts of polymerization and heat generation near the localized initiator zone and less elsewhere.
This unbalanced reaction profile may lead to process-related problems, such as high molecular weight material buildup near the initiator injection site, which may clog the injection port or the process fluid flow channel. It can also cause a buildup of high molecular weight material near the injection site or along the walls of the reaction tube that result in an occasional “sloughing off” of high molecular weight material. It can also lead, as previously mentioned, to ethylene decomposition. If a significant concentration of fresh initiator contacts the wall of the reactor tube in the reaction section (where temperatures are elevated), the initiator may decompose and quickly react, starting a localized reaction “hot spot” that may propagate into full blown system-wide decomposition.
Various attempts have been made to enhance the mixing of an injected material into a process fluid stream through various nozzle configurations and other system changes. Great Britain U.S. Pat. No. 1,569,518 (Kita, et al.) describes using mechanical restrictions—static inline mixers—to create a turbulent flow. U.S. Pat. No. 3,405,115 (Schapert, et al.) describes something akin to a sparger where gas streams are split, a catalyst is injected in one stream, and the gas streams recombined. PCT Patent Publication No. WO 2005/065818 (Hem, et al.) describe a non-circular reaction tube profile. U.S. Pat. No. 6,677,408 (Mahling, et al.) describes a dog-bone constriction with in-line blades used to generate two counter-spinning gas flows upstream of an initiator injection site. U.S. Pat. No. 6,951,908 (Groos, et al.) has “swirl elements” for introducing initiator into the reaction system. European Published Application No. 0449092 (Koehler, et al.) describes an general injection nozzle.